1. Field of the Invention
This invention relates generally to sump suction pumps and more particularly to this type of pump as utilized with a liquid metal cooled nuclear reactor.
2. Description of the Prior Art
A nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and are assembled within a nuclear core. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactors usually comprise one or more primary flow systems and a corresponding number of secondary flow systems to which conventional large steam turbines and electrical generators are coupled. Thus, a typical energy conversion process for commercial nuclear reactors involves transfer of heat from the nuclear core to a primary coolant flow system then to a secondary coolant flow system and finally into steam from which electricity is generated.
In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, a reactor coolant such as liquid sodium, is circulated through the primary coolant flow system. Typically, the primary system comprises a nuclear core within a reactor vessel, a heat exchanger, a circulating pump and piping interconnecting the aforementioned apparatus. In nuclear reactors having more than one primary system, the nuclear core and the reactor vessel are common to each of the primary systems.
The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the nuclear core. The heated reactor coolant exits from the reactor vessel and then flows to the heat exchanger where the heat previously acquired is transferred to the secondary flow system associated therewith. The cooled reactor coolant exits from the heat exchanger and flows to the pump which again circulates the coolant into the pressure vessel repeating the described flow cycle.
The circulating pump utilized with the above-described primary coolant flow system is classified, in the nuclear art as a "cold leg" pump. This is because the pump is located downstream of the heat exchanger where the reactor coolant is relatively cool. In comparison, a "hot leg" pump is one which is located immediately downstream of the reactor vessel where the reactor coolant is relatively hot. Thus, a cold leg pump operates within a cooler and hence, less hostile environment. From a design standpoint, the cold leg pump is therefore more desirable. However, because of a phenomenon known as "drawdown", a cold leg pump relinquishes some of its advantages.
Drawdown refers to the change in liquid level of the liquid in a sump suction pump when the pump increases its speed from zero to full operating conditions. The amount of change of liquid level is equal to the friction head loss, in feet, between the reactor vessel outlet and the pump inlet. Since the friction head loss is much greater in a cold leg pump than in a hot leg pump, the liquid level change, or drawdown, is much greater in the former than the latter type of pump. The effect of pump drawdown is reflected in the length of the drive shaft connecting the pump impeller to the pump motor. For purposes of comparison, in one liquid metal system, the required length of the pump shaft is 12 feet for a hot leg pump and 36 feet for a cold leg pump. It is readily understandable therefore, that recent prior art have disclosed apparatus whereby the pump drawdown in a cold leg pump is minimized or eliminated completely. One such example of this prior art is found in the afore-referenced ERDA patent application.
The sump suction pump in the ERDA patent application is provided with a divider plate which divides the pump sump tank into an upper and a lower chamber. The pump impeller as well as the pump inlet nozzle and the pump outlet nozzle are located within the lower chamber. The chamber above the divider plate is partially filled with a reactor coolant and includes a cover gas above the level of coolant therein. The sump tank is sealed at its upper end by a cover plate. The pump motor is mounted on top of the cover plate. A drive shaft extends from the pump motor through the cover plate and into the sump tank where it is attached to the impeller below the divider plate. In this manner, the pump motor is protected against the corrosive effects of the liquid metal reactor coolant.
In operation, the divider plate allows a small amount of reactor coolant to leak from the upper chamber into the lower chamber of the sump tank. Therefore, when the pump increases its operating speed, the usual drawdown effect is limited to the level change as allowed by the leakage past the divider plate. This permits time for the pump cover gas pressure to be lowered to maintain an essentially constant level of liquid in the sump tank.
To illustrate the principle involved in the referenced application, assume a representative system friction loss from the reactor vessel outlet to the pump inlet of 15 psi at full coolant flow. Further, assume the cover gas pressure above the constant free surface level in the reactor vessel is maintained at 18 psig throughout the operating range. The cover gas pressure above the free surface level in the pump sump tank is separately controlled to maintain a constant level. At zero flow conditions, the level in the reactor vessel will be equal to the level in the pump sump tank and both cover gas systems will be at the same pressure of 18 psig. As flow is increased, the level of coolant in the pump sump tank will begin to fall. However, the divider plate will limit the rate of drawdown and allow time for the cover gas pressure in the pump sump tank to be readjusted and maintain a constant level. At full flow conditions, both free surface levels will still be equal. The gas pressure in the reactor vessel will remain constant at 18 psig but the pressure in the pump sump tank will be close to atmospheric at 3 psig.
This recent prior art has significantly advanced the state of the art of cold leg pumps for use with liquid metal cooled nuclear reactors. However, this is not to imply that the recently disclosed art is not without its difficulties. For example, as explained above two separate cover gas control systems, operating in parallel, are required to maintain the free surface levels of reactor coolant in the reactor vessel and in the sump tank. Thus, there exist a continuing need for improvements in the art of sump suction pumps.